Electrolytic chromate production



Nov. 2, 1943. w. J. KNOX, JR, EI'AL 8 ELECTROLYTIC CHROMATE PRODUCTION- Filegi June 16, 1939 IRON ANOL YTE 61 CATHOLYTE SUPPLY CATHOL Y7! STORAGE TANK CHROMATE serum/s TANK 21 Patented Nov. 2, 1943 ELECTROLYTIC CHROMATE PRODUCTION William J. Knox, Jr., Hammond, Ind., and Richard A. Kelly, Chicago, 111-, assic'norsto International smelting & Refining Company, New York, N. Y., a corporation of Montana Application June 16, 1939, Serial No. 279,444 3 Claims. (01. 204-89) This invention relates to the production of metal chromates, and has for its object the provision of certain improvements in the production of metal chromates by electrolytic methods.

The invention is more particularly concerned with the production of metal chromates in a bifiuid cell in which the electric current passes from a soluble anode of the metal through an anolyte capable of serving upon electrolysis as a solvent of the metal, a diaphragm-and a catholyte containing chromate ions to a cathode, and in such a process the invention aims to replace the chromate ions depleted during the production of the metal chromate with chromate ions added to the process in the form of a soluble metal salt'of chromic acid.

The improvements of the invention are particularly applicable to the electrolytic process for the manufacture of metal chromates described and claimed in the application for United States Letters Patents of Elbert F. Weaver Ser. No. 134,520, filed April 2, 1937, and Ser. No. 258,463 filed Feb. 4, 1939, now Patent No. 2,242,634, dated June 20, 1941. In that process, as applied to the production of lead chromate in a bifiuid electrolytic cell, a lead anode is immersed in the anolyte and an iron cathode is immersed in the catholyte, the two electrolytes being separated by the permeable diaphragm of the cell. The anolyte is preferably an aqueous solution of sodium acetate or sodium nitrate together with small amounts of sodium chromate with or without small amounts of sodium dichromate or sodium hydroxide. The catholyte is preferably an aqueous solution of sodium chromate with or without sodium dichromate or sodium hydroxide.

The production of lead chromate isaccomplished by passing an electric current between the anode and the cathode. is precipitated in the anolyte compartment and is separated from the anolyte solution preferably outside the cell. Means are provided for the circulation of both the anolyte andcatholyte solutions within the anolyte and catholyte compartments of the cell and to and from anolyte and catholyte storage tanks outside the cell. In practice a reservoir containing a source of suitable solution containing chromate ions-is provided. Means are provided whereby'this material is fed into the catholyte solution at such a rate as to maintain a constant concentration of chromate ions in this solution. Heretofore it has been found necessaryto use chromic anhydride or chromic acid as a source of chromate ions. If for example sodium dichromate or sodium chromate were used as a source of chromate ions, the accumulation of sodium ionsv as the process continued would soon render the solutions worthless and would prevent proper control of the composition of the resulting cell product. This The lead chromate.

condition will be better understood by considering the following reactions which probably take place in the operation of the electrolytic lead chromate process.

Under the influence of the electric current the sodium nitrate present in the anolyte solution reacts with the lead anode forming lead nitrate:

the cathode and form sodium hydroxide,liberat- Simultaneously basic lead chromate is formed by a reaction which occurs in the anolyte compartment between the lead nitrate, the sodium chromate and the sodium hydroxide present therein:

The chromate and hydroxyl ions required for these latter reactions come from the catholyte through the diaphragm by migration. In case the operation of the cell is such that the hydroxyl ion concentration contained in the anolyte is sufficient low, 1. e., if little or no free sodium hydroxide is present, the following reactions will occur resulting in the formation of normal lead chromate.

2Pb (N 03) z+2NazCrO4 2PbCr04+ 4NaNO3 These two latter reactions serve to illustrate that by proper regulation of the concentration of the hydroxyl ions contained in the anolyte solution a cell product of a predetermined chemical composition may be produced. A convenient method of controlling the composition of the anolyte solution, and thus indirectly of the cell product, is by controlling the composition of the catholyte solution. For this reason it is necessary that the hydroxyl ion concentration of the The acid in turn reacts with free sodium hydroxide or sodium chromate, if no free sodium hydroxide is present, to form water and either sodium chromate or sodium dichromate.

HzCrO4+NaaCrQ4Na2Cr207-i-H2O The only raw'materials consumed in the process are lead, chromic acid and water. Since the process is continuous and since the solutions used are not discarded but are continuously circulated through the apparatus, it has heretofore been customary to add the chromic acid to the process in the form of chromic anhydride. Chromic anhydride (chromic trioxide) is usually prepared from a sodium salt of chromic acid by treatment with concentrated sulphuric acid. This treatment with sulphuric acid, together with the succeeding treatments necessary to remove the sodium sulphate formed, obviously increases the cost of the chromic anhydride as compared with that amount of the original sodium salt of chromic acid containing an equivalent amount of chromic acid. This comparison is illustrated by the following costs prevailing at the present time:

Chromic anhydride, cost cents per pound,

per pound of contained CrOa.

Sodium dichromate, cost 6.25 cents per pound, contains 67.11% 0103; giving a cost of 9.31 cents per pound of contained CrOa.

Substantial economic savings could therefore be realized if it were possible to add chromic acid to the process in the form of some soluble salt such as sodiumdichromate. However, as hereina method for removing from the solutions the excess of sodium (or other metal) ions in order to maintain the concentrations of the chromate ions, dichromate ions and sodium ions at constant and predetermined values. In accordance with the invention the excess metal ions are reinoved from the solutions by electrochemical transference through a permeable diaphragm in a separate electrolytic cell. The excess metal ions may be removed from either the anolyte or catholyte of the lead, or other metal, chromate cell, but for the purposes of illustration we shall confine the description of the invention to the regeneration of the catholyte.

The invention may advantageously be carried out in a bifluid electrolytic cell which we shall designate as the regeneration cell. Such a cell is divided into .an anolyte and a catholyte compartment by a diaphragm of some suitable permeable material such as fabric. An anode is suspended in the anolyte compartment in direct contact with the solution contained therein. The anode is of iron or some other electrical conductor, preferably a metal, which i insoluble and passive in the solution contained in the anolyte compartent. The cathode is suspended in the catholyte compartment in direct contact with the solution therein. The cathode may be constructed of iron or any electrical conductor, preferably a metal, which is not corroded by the solution contained in the catholyte compartment.

The solution contained in the anolyte com- Ill - contains 99.5% CrOa; giving a cost of 15.07 cents solution may contain, in addition, the water soluble salts of one or more acids provided the metal ions contained will form water soluble hydroxides and provided these saltsdo not react with each other or with the salt of chromic acid to yield a product insoluble in water. The solution may contain an excess of the acid of any or all of the salts present or an excess of the hydroxide of any or all of the metal ions present.

We have found it convenient to utilize the solution circulated through the catholyte compartment of the lead (or other metal) chromate cell in the anolyte compartment of the regeneration cell. This solution contains sodium chromate and either sodium acetate or sodium nitrate. It may or may not contain sodium sulphate and either sodium dichromate or sodium hydroxide. It may or may not contain an excess of one or all the acids of the salts present.

The solution contained in the catholyte compartment of the regeneration cell consists chiefly of an aqueous solution of the hydroxides oi the metal ions present in the solution contained in the anolyte compartment of the same cell. In

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partment of the regeneration cell may be an practice, small amounts or the salts present in the solution in the anolyte compartment are also present in the solution in the catholyte compartment but it is essential to the economy of the process that these quantities be small. In practice it is convenient that the solution contained in the catholyte compartment of the regeneration cell be an aqueous solution of sodium hydroxide.

Means are provided to maintain a circulation of the solution within the anolyte compartment of the regeneration cell and to and from suitable supply tanks. It has been found advantageous to make this circulation system an integral part of the catholyte circulation system provided for the lead (or other metal) chromate cell.

Means are provided to maintain a circulation of the solution within thecatholyte chamber of the regeneration cell. We have 'found it convenient to feed water into the catholyte compartment by suitable means at a constant rate.

The sodium hydroxide content is maintained by electrolysis, the sodium hydroxide solution being discharged from the catholyte compartment by suitable means. This sodium hydroxide solution may be used for any purpose which may prove convenient, We have found that it may be used advantageously in the final treatment of the basic lead (or other metal) chromates produced in the main process.

The anode and cathode of the regeneration cell are connected to a suitable source of electric current such as a generator. Upon passage of an electric current between the anode and the cathode gases are liberated at the anode and the cathode. The gas liberated at the anode is oxygen; the gas liberated at the cathode is hydrogen. The passage of an electric current through a solution of electrolytes is dependent upon the migration of the ions present. Positive or metal ions present such as the sodium ions move from the anode towards the cathode. Negative or acid ions present such as hydroxyl ions, chromate the cathode of the regeneration cell the net result is a continuous passage of the metal ions present 7 from the anolyte compartment through the diaphragm to the catholyte compartment. In a like manner the movement of the acid ions away from the cathode tends to minimize any loss caused by diffusion of these ions through the diaphragm.

The following reactions probably take place in the operation of the regeneration cell. At the anode water is decomposed and gaseous oxygen and hydrogen ions are set free:

The sodium ions present in the anolyte compartment migrate towards'the cathode passing through the diaphragm. At the cathode the sodium ions react with water liberating hydrogen gas:

This migration of the' sodium ions probably represents the mechanism whereby a portion of the electric current is passed through the solutions. In a like manner the balance of the electric current is passed by the migration of the other ions present. The sodium ions, hydrogen ions, and other metal ions present migrate towards the cathode while the hydroxyl ions and acid ions present migrate towards the anode. This results in an accumulation of the metal ions in the catholyte compartment of the regeneration cell. In the meantime the migration of the acid ions toward the anode tends to prevent loss of the acid ions from the solution contained in the anolyte compartment.

In the single figure of the accompanying drawing, we have diagrammatically illustrated, merely by way of example, a suitable apparatus or system for the practice of the present invention in conjunction with the production of lead chromate in accordance with the process described in the from this washing solution is also returned to tank 35 to provide the makeup solution. The washed lead chromate. product is passed to a drying chamber 23 and thence to a grinding mill 24 to prepare it for the market. A pump 26 serves to return the anolyte solution from the anolyte storage tank 20 through a conduit 21 to the anolyte compartment l l of the lead chromate cell II] at a rate sufficient'to balance the rate at which anolyte solution and cell product are withdrawn from the anolyte compartment.

Catholyte solution is fed from a catholyte supply tank 5| through a valved conduit 52 to the bottom of the catholyte compartment E2 of the lead chromate cell III. From thence the catholyte overflows through the overflow conduit into a launder 3 I, and flows by gravity through a conduit 32 into the catholyte discharge storage tank 33.

The chromate ion concentration of the cath olyte is decreased and the hydroxyl ion concen-. tration is increased during passage through the lead chromate cell l0. To restore the chromate ion concentration of the catholyte, a valved conduit 34 is provided to supply sodium dichromate from a suitable storage tank 35. The sodium dichromate is stored in the tank preferably as a concentrated aqueous solution.

aforementioned Weaver patent applications.

The apparatus consists generally of a bifluid electrolytic cell I 0 for the production of lead chromate and a second bifiuid electrolytic cell 53 for the regeneration of chromate liquors in accordance with the principles of the invention.

The lead chromate cell H! is divided into an anolyte compartment H and a catholyte com-' partment l2 by a diaphragm l3 of fabric or other suitable permeable material. A lead anode I4 is suspended in the anolyte compartment in direct contact with the anolyte, and an iron cathode I5 is suspended in the catholyte compartment in direct contact with the catholyte. The catholyte in the compartment I2 is preferably maintained at a slight hydrostatic head with respect to the anolyte in the compartment H. A generator G is electrically connected to the anode and to the cathode.

A valved conduit l6 communicates with the bottom of the cell I0 and serves to withdraw the anolyte solution together with'the precipitated lead chromate into the settling tank II. From thence the clear anolyte overflows into launder l8 and through conduit I9 to the anolyte storage tank 20. a The thickened lead chromate pulp is withdrawn from the tank I! through a valved conduit 2| to a vacuum drum filter 22. The filtrate from thefilter 22 is returned through a conduit 25 to the anolyte storage tank 20. The

. thickened pulp from the filter 22 is washed with water to recover the electrolyte salts entrained therein. When required a portion of the filtrate To permit proper adjustment of the chromate ion concentration, and either the dichromate ion concentration or the hydroxyl ion concentration of the anolyte solution, conduits 40 and 39 provide a means of withdrawing sodium dichromate fromthe storage tank 35 and adding it directly to the anolyte solution contained in the anolyte storage tank 20. A conduit 38 provides a means of withdrawing catholyte from the catholyte storage tank 33 and adding it to the anolyte contained in the anolyte storage tank 20.

A pump 36 provides for the passage of the catholyte solution contained in the catholyte storage tank 33 through a conduit 31 into the anolyte compartment 53 of'the regenerating cell 50. From thence this solution is withdrawn through a valved conduit 64 into the catholyte supply tank 5|. The regeneration cell 50 is divided into an anolyte compartment 53 and a catholyte compartment 54 by a diaphragm 51 of fabric or other permeable material. An anode 63 of iron or other suitable insoluble conducting material is suspended in the anolyte compartment 53 in direct contact with the anolyte therein. The anolyte in the. anolyte compartment 53 of the regeneration cell is preferably the catholyte circulated through the lead chromate cell II). A cathode 55 of iron or other suitable insoluble conducting material is suspended in the catholyte compartment 54 in direct contact with the catholyte therein. The catholyte in the compartment 54 is preferably maintained at a slight hydrostatic head with respect to the anolyte in the compartment 53. A suitable source of electric current such as a generator 62 is connected to the anode 63 and the cathode 55. It has been found entirely practical to use the same source of electric current for the lead chromate cell I0 and the regeneration cell 50. In such a. case generators G and 52 may be replaced by a single generator of sufiicient capacity to supply electric current to both cells.

A supply of water is contained in a. supply tank 58. This water is fed through a valved conduit 56 to the bottom of the catholyte compartment 54 of the regeneration cell 50. During the rise of the water upward through the catholyte compartment a concentration of sodium hydroxide is attained. The aqueous solution of sodium hydroxide overflows the catholyte compartment through an overflow conduit 60 into a launder 59, and thence into a discharge conduit Bl. aqueous solution of sodium hydroxide may be used to advantage in treating the lead chromate pulp prior to its introduction into the drier 23, or may be otherwise utilized or discarded as desired.

We have found it convenient to express the amount of sodium hydroxide accumulated in the catholyte compartment 54 of the regeneration cell 50 in units of electric current flow. One faraday of current may be considered equivalent to 40.00 grams of sodium hydroxide. We have found it practical to operate the cell in such a manner that an amount of sodium hydroxide is formed in the catholyte .compartment equivalent to from 30 to 45% of the current passed.

We have found this value together with the voltage of the cell to be of importance in governing the efiiciency of the regeneration process. We have found that the following factors-have a definite effect upon the efilciency of the regeneration process:

A. Current density at the cathode 13. Temperature of the solutions C. The sodium hydroxide concentration of the solution in the catholyte compartment.

D. The permeability of the diaphragm material static head eifective at the diaphragm F. The spacing between anode and cathode.

We have found the regeneration cell to operate successfully when these factors are held within the maximum limits stated below but we prefer to operate the cell in such a manner that the factors are within the more confined limits which are stated below:

A. The current density at the cathode when expressed as current flow per square foot of surface may be as small as 5 amperes and as great as 100 amperes. However, we prefer to operate with current densities of -30 amperes per square foot.

B. The cell may be operated with the temperature of the solutions contained as low as 10 C. and as high as 70 0. However, we prefer to operate with the temperature of the solutions no lower than C.-and no higher than 50 C.

C. The sodium hydroxide concentration in the catholyte compartment may be as low as 1 gram per liter or it may be as high as 100 grams per liter. However, we find thatmost efficient operation may be obtained if the sodium hydroxide concentration is no less than 8 grams per liter or no higher than 20 grams per liter.

D. The diaphragm material should be of sufficient permeability to permit passage of electric current without excessive voltage losses but it should not be so porous as to allow. too great a diffusion of the adjoining solution-s.

E. We have found that successful operation of the regeneration cell may be obtained with a slight hydrostatic head at the diaphragm on the anolyte side but we prefer to operate in such a manner that a slight hydrostatic head is maintained in the catholyte side of the diaphragm.

The.

E. The magnitude and direction of the hydro- F. We have found it desirable to maintain a eration cell. We have found it impractical at the present time to make this distance much less than one half inch.

The following examples embody data collected in carrying out the regeneration of chromate solutions in the manner previously described and serve to illustrate more or less typical practices of the regeneration process of the invention.

'Ewample 1 The electrolysis was carried out in a small bifluid cell. The anode and cathode were each made of iron and each exposed 0.148 square foot of surface. The diaphragm 5'! consisted of fabric. Means were provided to circulate a solution containing 4.0% sodium nitrate, 2.3% sodium dichromate, and 2.5% sodium sulphate through the anolyte compartment 53 and to discharge the same into a suitable vessel. A slow stream of water was fed into the catholyte compartment 54 and. the solution discharged was collected in a suitable vessel. Electrolysis was continued for 3.25 hours. During this period an average current of 5.25 amperes was maintained between the anode and the cathode. The average voltage required during the period was 4.33 volts. At the end .of the period the solution discharged from the catholyte compartment contained 10.61 grams 'of sodium hydroxide.

Example 2 The electrolysis was carriedout in a bifluid cell. The anode and cathode each consisted of iron sheets 4.15 square feet in surface area. The diaphragm used consisted of fabric. Means were provided to circulate a solution containing 5.0% sodium nitrate, 4.6% sodium chromate, and a small excess of sodium hydroxide through the anolyte compartment 53 and to discharge the regenerated solution into a suitable vessel. Fresh water was fed to the catholyte compartment 54 and the solution discharged from this compartment was collected in a suitable vessel. Electrolysis was continued for 16.33 hours at an average current flow of amperes and an average voltage of 3.81 volts. Prior to passage of the chromate solution into the anolyte compartment an amount of sodium dichromate equivalent to 1749 grams of chromic acid was added in a manner such as would be used in practice to replace the chromate ions depleted during the production of lead chromate. After electrolysis the solution discharged from the catholyte compartment was found to contain 758 grams of sodium hydroxide and 10.61 grams of sodium chromate.

The preceding examples serve to illustrate the regeneration process as an improvement to the electrolytic process for producing lead, or other metal, chromates. We have found less than 0.7% of the chromic acid introduced as sodium dichromate into the chromate solution is lost in the sodium, hydroxid solution discharged from the catholyte compartment 54. This improved process of producing lead chromates makes possible a very good overall. recovery of chromate liquors. We have found that sodium dichromate may be used in place of chromic anhydride as a source of chromate ions at an additional power expenditure of 1.25 kilowatt hours to remove the sodium hydroxide combined with each pound of chromic anhydride. When this power expenditure is compared with the difierence in price between sodium dichromate and chromic anhydride the economic savings made possible by the invention ar apparent. I

We have further found that the improved methscription that the catholyte in the lead (or other,

metal) chromate cell i is depleted of chromate ions and enriched in metal (e. g. sodium or other alkali-metal) ions as a result of the electrolytic action. In other words the catholyte withdrawn from the compartment l2 contain per unit volume less chromate ions and more metal ions than contained in the sam unit volume of the catholyte as supplied to the compartment l2. The depletion of chromate ions is replaced by adding a soluble metal salt of chromic acid (e. g. sodium dichromate) to the withdrawn catholyte at any appropriate stage in its regeneration, as for example prior to the introduction of the withdrawn catholyte into the renegeration cell 50. Such an amount of the soluble metal salt of chromic acid is added to the withdrawn catholyte as is required to replace the depletion of chromate ions therein. (In the regeneration cell 50 the excess of metal (e. g. sodium) ions is removed by the electrolytic treatment in the course of which the am. lyte (in compartment 53) is depleted or impoverished in metal ions and th catholyte (in compartment 54) is enriched, that is become more concentrated, in the hydroxide of the metal (e. g. sodium hydroxide). The regeneration of the catholyte withdrawn from the lead (or other metal) chromate cell l0 thus involves two principal steps (1) the electrolytic removal therefrom of the excess of metal (e. g. sodium) ions and (2) the addition thereto of chromate ions in.

the form of a soluble metal salt of chromic acid. This removal of excess metal ions and addition of chromate ions is so controlled and proportioned as to maintain the concentrations of chromate ions, dichromate ions and sodium ions in the solutions of the chromate producing cell It) at substantially constant predetermined values.

We claim:

1. In the electrolytic production of chromates= tially constant the chromate ion concentration of said catholyte and which comprises withdrawing catholyte from said cell and removing from the withdrawn catholyte by electrolysis such an amount of alkali-metal ions as is equivalent to the aforesaid depletion of chromate ions, returning the catholyte from which the alkali-metal ions were removed to the catholyte compartment of the electrolytic cell, and restoring to the catholyte in the form of an alkali-metal salt of chromic acid such an amount of chromate ions as is equivalent to the aforesaid depletion of chromate ions.

2. In the electrolytic production of chromates in an electrolytic cell having an anolyte and a catholyte separated by a permeable diaphragm wherein the catholyte becomes depleted in chromate ions and enriched in alkali-metal ions as a result of the electrolytic action, the improvement which is characterized by utilizing an alkali-metal salt of chromic acid to maintain substantially constant the chromate ion concentration of said catholyte and which comprises withdrawing catholyte from said cell and removing from the withdrawn catholyte by electrolysis in a separate bi-fiuid cell in which th withdrawn catholyte is the anolyte such an amount of alkali-metal ions as is equivalent to the aforesaid depletion of chromate ions, returning the catholyte from which the alkali-metal ions were removed to the catholyte compartment of the first-mentioned electrolytic cell, and restoring to the catholyte in the form of an alkali-metal salt of chromic acid such an amount of chromate ions as is equivalent to the aforesaid depletion of chromate ions. a

3. In the electrolytic production of chromates in an electrolytic cell having an anolyte and a catholyte separated by a permeable diaphragm wherein the catholyte becomes depleted in chromate ions and enriched in alkali-metal ions as a result of the electrolytic action, the improvement which is characterized by utilizing an alkali-metal salt of chromic acid to maintain substantially constant the chromate ion concentration of said catholyte and which comprises withdrawing catholyte from said cell and removing from the withdrawn catholyte by electrolysis in a separate bi-fluid cell in which the withdrawn catholyte is the anolyte such an amount of alkalimetal ions as is equivalent to the aforesaid depletion of chromate ions, adding to the withdrawn catholyte so electrolyzed for the removal of alkali-metal ions such an amount of an alkalimetal salt of chromic acid as is equivalent to the aforesaid depletion of chromate ions, and returning the catholyte replenished in chromate ions to the catholyte compartment of the firstmentioned electrolytic cell.

WILLIAM J. KNOX, Ja. RICHARD A. KELLY. 

